1. Field of the Invention
This invention relates to recovering helium from gas streams obtained from natural gas reservoirs containing minor amounts of helium, major amounts of hydrocarbons, and significant quantities of nitrogen, by utilizing the Mehra process for initial purification of the gas streams.
2. Review of the Prior Art
A significant amount of helium is often found in natural gas reservoirs, but the natural gas is frequently contaminated with fairly large amounts of nitrogen which may have originated naturally or may have been injected into the reservoirs in suitable formations, such as in the central and north Texas areas of the United States, as part of an enhanced oil recovery operation or an enhanced gas recovery operation.
Such contamination by nitrogen and helium, if present, has caused the oil producer to curtail oil production because government regulations prevent him from burning the nitrogen-rich associated gas, and both environmental laws and a desire to preserve valuable resources prohibit him from venting the associated hydrocarbons. The oil producer is thus limited by the choice of technology available to him for properly processing the associated gases from an oil well. Most prior art technology, which involves cryogenic principles, cannot economically process the natural gas streams which contain more than 3 mol % nitrogen, even after subsidization with the revenue from oil production.
U.S. Pat. Nos. 4,623,371, 4,832,718, and 4,883,514 disclose several extractive-flashing embodiments of the Mehra process for countercurrently contacting a nitrogen-rich feed gas stream, which may range from lean to rich in hydrocarbons content, at any pressure with a lean physical solvent, using an extractor column, at least one flashing stage, and optionally a regenerator column, to produce a methane-rich gas product, meeting the pipeline specification for nitrogen content, and a nitrogen product. Any helium that is present in the natural gas is in the nitrogen product and is similarly concentrated.
In March 1991, the Gas Research Institute issued Report GRI-90/0290, entitled "Comparison of the Mehra Process for Nitrogen Rejection to a Cryogenic Process for Nitrogen Rejection from Subquality Natural Gas". In this report, GRI confirmed the technical feasibility and economic benefits of the Mehra concept which operates under non-cryogenic temperatures and requires inexpensive carbon steel metallurgy. The Mehra process requires 45% less compression and 30% less energy overall and also requires 12% less capital than a comparable cryogenic process.
For many years, the helium has been extracted from helium-containing natural gas by cooling the gas stream in a cryogenic nitrogen rejection unit (NRU) to a temperature below the liquefaction points of its hydrocarbon constituents but above the liquefaction point of helium. Because both helium and nitrogen are highly volatile, helium tends to become concentrated in nitrogen recovery streams, such as in the nitrogen stream when nitrogen and methane are separated. The resulting product of this separation is called crude helium and generally comprises about 50-70% helium and 50-30% nitrogen. This crude helium is either stored for future use or is further purified to a marketable grade (99.995% purity).
Helium has been used for years as a lighter-than-air material for balloons, dirigibles, and the like and has been increasingly utilized for welding because of its chemical inertness and as a coolant for superconductivity applications.
Helium has conventionally been recovered and purified by stand-alone cryogenic processes, such as described in U.S. Pat. No. 3,599,438. Because cryogenic processes are costly, however, they have frequently been combined with adsorption processes, such as those described in U.S. Pat. Nos. 3,653,220, 4,192,661, and 4,238,211, using temperature swing adsorption, as described in U.S. Pat. Nos. 3,616,602 and 3,683,589, or pressure swing adsorption, as described in U.S. Pat. Nos. 4,659,351, 4,666,481, and 4,701,200, for regenerating the adsorbent.
Helium has also been separated from other volatile gases by permeation through thin, non-porous membranes which exhibit good selectivities for oxygen, helium, nitrogen, and carbon dioxide over other gases in gas mixtures, as described in U.S. Pat. No. 4,666,468.
In addition, U.S. Pat. No. 4,690,695 discloses the combination of one or more permeable membranes for bulk separation and for residual product gas recovery with a pressure swing adsorption (PSA) process for the recovery of high purity product gas, such as helium, the waste gas from the PSA system being passed to one or more such permeable membranes for enhanced product recovery, the recovery levels achieved being advantageously reconciled with the corresponding compression and other cost factors pertaining to the overall process for the production of such high purity product gas. Semi-permeable membrane units have been hybridized with pressure swing adsorption (PSA) units as described in U.S. Pat. No. 4,701,187 to produce purified components (99+%) at high recovery (80+%) from gas mixtures containing at least one other component because stand-alone membrane units were found to be inefficient for such high purity/high recovery products, while stand-alone adsorption units were found to be very effective in producing a purified gas stream if their feed streams were relatively pure (e.g., 70%).
U.S. Pat. No. 4,717,407 discloses a process for recovering a helium-rich stream from a helium-containing gaseous feed mixture which may be natural gas, a slip stream from a nitrogen rejection unit, or crude helium at pressures varying from atmospheric to more than 3,000 psia and at helium concentrations ranging from 0.1-90 mol %. This process recovers high-purity helium (i.e., greater than 95 mol %) and/or crude helium (i.e., 40-70 mol %).
The helium-containing gaseous feed mixture is fed to a non-membrane separation unit to produce a helium-enriched stream and a helium-depleted stream. This unit can be an adsorption, absorption, cooling, or partial condensation and/or rectification type unit. The purpose of the non-membrane unit is to alter the relative content of helium in relationship to the nitrogen content, whereby a helium-enriched stream leaves overhead, when it is an absorption unit, and a helium-diluted stream, enriched in nitrogen, leaves from the bottom of the unit.
At least a portion of the helium-enriched stream is fed to a membrane unit consisting of a cascade of membranes with an internal recycle stream, the helium-enriched stream being separated by the membranes into a helium-rich permeate stream and a helium-depleted reject stream. To obtain high-purity helium, the permeate stream is then fed to a pressure swing adsorption (PSA) unit from which a helium product of 99.99 mol % purity is obtained.
The process described in U.S. Pat. No. 4,717,407 appears not to be adapted, however, for receiving nitrogen-rich gas streams having low concentrations of helium (less than 5.0 mol %) from a nitrogen rejection unit, such as the Mehra process described in U.S. Pat. Nos. 4,623,371, 4,832,718, and 4,883,515, unless such a low-helium product stream from the nitrogen rejection unit is fed to a first membrane unit and then to a non-membrane device for further processing before final treatment in a second membrane unit, as illustrated in FIG. 4 thereof.
A need consequently exists for a method of combining the Mehra nitrogen rejection units with membrane and PSA units for producing high-purity helium from low-purity (below 5.0 mol % helium) or subquality natural gas.